This invention relates to a semiconductor switching device.
Conventionally, a compact four-layer diode or silicon symmetrical switch is employed for a triggering circuit for use in a fluorescent lamp, an igniting circuit for application to as gas lighter or gas appliance, etc. Such diode or switch is so designed as to attain a constant breakover voltage V.sub.B, a large holding current I.sub.H ; and a small breakover current I.sub.B flowing upon application of the breakover voltage V.sub.B.
An example of such type of silicon symmetrical switch is shown in cross-section in FIG. 1. This switch includes an n conductivity type layer 1, and n conductivity type layer 2, a p conductivity type intermediate layer 3, an n conductivity type intermediate layer 4, and a p conductivity type intermediate layer 5. The n conductivity type intermediate layer 4 is so formed as to have a thickness of approximately 95.mu., while each of the p conductivity type intermediate layers 3 and 5 is so formed as to have a thickness of approximately 15.mu.. Further, these p conductivity type intermediate layers 3 and 5 are each so formed as to have a maximum impurity concentration of about 4.times.10.sup.17 atoms/cm.sup.3 at the proximity of the respective junction faces with the n conductivity type layers 1 and 2.
FIG. 2 shows the profile of impurity diffusion in the layers 1 to 3 of the switch shown in FIG. 1. In this Figure, on the ordinate is plotted the impurity concentration (atoms/cm.sup.3) of each layer, while on the abscissa is plotted the depth (.mu.) of impurity diffusion in each layer as measured from the upper surface of the layer 1.
As seen from FIG. 1, the silicon symmetrical switch may control current flow in both directions, or in forward and reverse directions with substantially the same electrical characteristics. Table 1 below shows the electrical characteristics of each of ten samples of the switch both forward and reverse directions.
TABLE 1 ______________________________________ For- Re- For- Re- For- Re- ward verse ward verse Switch ward verse I.sub.B I.sub.B I.sub.H I.sub.H No. V.sub.B (V) V.sub.B (V) (.mu.A) (.mu.A) (mA) (mA) ______________________________________ 1 116 115 5 5 80 80 2 112 112 3 3 69 65 3 119 119 1 1 30 24 4 115 116 1 1 30 20 5 115 115 2 2 65 65 6 110 118 1 1 66 70 7 112 115 2 2 69 59 8 110 112 8 8 81 79 9 115 115 2 2 23 23 10 110 112 2 2 52 48 ______________________________________
As seen from the above Table 1, in the silicon symmetrical switch shown in FIG. 1 the holding current I.sub.H can be kept as relatively large as 20 to 80 mA and yet as compared with this holding current the breakover current I.sub.B allowed to flow into the switch upon application of the breakover voltage thereof can be chosen to be as appreciably small as 1 to 80 .mu.A. This is one of the big merits. However, where it is desired to enhance the responsive characteristics of the switch, the breakover current I.sub.B is required to be further reduced to a value equal to or less than, for example, 0.1 .mu.A.